London’s transportation agency on Thursday asked a court to make a decision on the legality of the smartphone app, which allows customers to order a ride from drivers who do not have licenses to pick up passengers who hail them on the street.

Officials at the agency, Transport for London, said they believe San Francisco-based Uber— whose rise is disrupting traditional taxi businesses in many parts of the world— is within the law but wanted a court to make a binding ruling.

Meanwhile, the Licensed Taxi Drivers Association, which represents 10,200 of the roughly 22,000 regulated black cab drivers in London, applied for court summonses against six drivers of privately hired cars, known as minicabs, for using the app to find fares. Steve McNamara, the association’s general secretary, said enough evidence had been gathered against the six, though he declined to explain how the drivers were selected or what the evidence was.

McNamara said the drivers’ angst was directed at the transport agency for not enforcing the rules.

“They’re frightened of Uber’s money, they’re frightened of Uber’s influence, they’re frightened of the amount of lawyers and stuff that Uber are going to throw at them,” McNamara said.

TFL couldn’t be reached for a response.

The developments on Thursday came two weeks before a planned mass protest by black cab drivers, who have pledged to park up to 15,000 black cabs near Trafalgar Square and bring the city of 8 million to a standstill.

“It will be quite a visual sight, all those cabs in one place,” McNamara said. “I think it’s going to make quite an impact. It will certainly gridlock London for the day.”

The clash in London is similar to fights across Europe. In France, the government is considering new rules in an effort to balance competition but scrapped a proposal to force drivers using Uber to wait 15 minutes before picking up their fare. In Brussels, Uber is banned and unlicensed drivers face a €10,000 ($13, 600) fine for picking up passengers.

Uber said it welcomed the chance to get a court ruling.

“Uber fully share the vision that technology should be a key driver in changing the way people are moving around their city, improving the experience for both riders as well as drivers,” Jo Bertram, the company’s general manager in London, wrote Thursday in a company blog post.

A central question is whether the Uber app used by drivers to calculate fares should be considered a fare meter. U.K. law says unlicensed taxi drivers can’t use a taximeter, which the law defines as a “device for calculating the fare to be charged in respect of any journey by reference to the distance traveled.” Minicabs set prices in advance and don’t use a meter during the ride.

The transportation agency said it scrutinized Uber in its “largest ever compliance investigation” and found the record-keeping was satisfactory but there were “certain technical aspects” of the operating model Uber would need to address.

“On the issue of taximeters, the law is unclear and we have taken a provisional view,” said Leon Daniels, TFL’s managing director of surface transport. “This is the sensible approach, and we hope that London’s taxi drivers and private hire drivers and operators will work with us to bring clarity on this issue.”

Kit Malthouse, the deputy mayor for business and enterprise, said he generally supported Uber’s entrance to the market, but that licensed taxi drivers have legitimate concerns about fairness and public safety.

“This is a difficult one for cities around the globe,” he said. “You can’t have it both ways. Either we all have the same regulation or we’ll all have no regulation. Which would you like? And I think what we’re seeing at the moment is that question being writ large by the taxi drivers.”

Malthouse said that in the past, regulations helped minicabs, which aren’t allowed to pick up street hails but must be separately booked. He said it was also important for the government to be able to have an accurate way to tax the companies.

Uber is one of a number of taxi apps in London, but the taxi association says most others play by the rules. GetTaxi and Hailo, for example, allow potential passengers to electronically hail black cabs. But Hailo recently applied for a private hire license, which would open it up to minicab drivers, saying that there weren’t always enough black cab drivers available.